The present invention relates generally to mobile communication devices such as mobile telephones and the like, and more specifically to a system and method for reducing the Specific Absorption Rate (SAR) of a mobile communication device by controlling the average power of radio frequency electromagnetic energy radiated by the device when the device is in proximity to a human body.
Compact mobile telephones in common use today employ a radio transceiver (a combination of transmitter and receiver) that uses radio frequency electromagnetic waves to communicate with fixed installations, called base stations. Typically, these base stations are networked to a central switching station that directs mobile telephone calls to the desired location. In existing networks, each base station is capable of measuring the received signal strength from a mobile telephone. If the received signal is determined to be strong, for example, when the mobile telephone is close to the base station, the base station may advise the mobile telephone to utilize a lower power setting (i.e., to reduce the power of the radio frequency electromagnetic energy being radiated). In this manner, less battery power is consumed by the mobile telephone allowing longer talk time.
It is known that exposure to very high levels of radio frequency electromagnetic energy, particularly at microwave frequencies, can heat biological tissue causing harm by increasing body temperature, disrupting behavior, and damaging biological tissue. Consequently, the effect of low levels of radio frequency energy, such as produced by mobile telephones, on human tissue has also come under intense scientific investigation. While there is no scientific proof that the relatively low levels of radio frequency energy produced by mobile telephones can cause damage to human tissue, many governmental bodies, such as the United States Federal Communication Commission (FCC) and many European regulatory organizations, have nevertheless imposed precautionary regulations and measurement guidelines aimed at limiting the exposure of users to such energy.
Accordingly, before a mobile telephone can be marketed, it typically must comply with maximum heating capacity guidelines imposed by such governmental regulating organizations. For instance, in the United States and Europe, the telephone must meet various technical standards, including compliance with radio frequency exposure limits defined in terms of the telephone's specific absorption rate (SAR). SAR is a widely accepted measurement of radio frequency energy absorbed into the body in watts per kilogram (W/kg) averaged over a certain amount of tissue when the telephone is situated close to the human body.
In general, the SAR value of a mobile telephone depends on the average radiated power of the telephone, frequency (because of different antenna properties and material properties of the human tissue at different frequencies) and geometrical and mechanical configurations of the telephone. SAR values are most affected by the distance between any radiating structure to the human tissue and the average radiated power. The average radiated power depends on the communication standard utilized, and may vary within the standard depending on the traffic mode in use. Thus, for example, a GSM (Global System for Mobile Communication) mobile telephone will have different average radiated power levels due to a different number of used timeslots in the GPRS (General Packet Radio Service) data transfer mode and a different power level setting in terms of peak power in the used timeslots.
Existing concepts used for reducing the SAR value of a mobile telephone are based on three ideas: adding reflective shielding to the mobile telephone, creating distance between the antenna of the telephone and the human body, and reducing the radiated power of the telephone through incorporation of absorbing materials within the telephone's housing. However, these concepts do not account for different modes of operation of the mobile telephone. Further, they generally have a negative impact either on the design of the mobile telephone, or on the quality of the communication link between the mobile telephone and the base station. For instance, creating distance between the antenna of the telephone and the human body is contrary to typical requirements that the mobile telephone be extremely compact. Further, reflecting devices cut any radio frequency link in the direction of the shield even in situations where no human body is present (e.g., when the mobile telephone is being used as a modem). Such devices further reduce the ability of the telephone to maintain an existing link when the user alters the telephone's orientation. Similarly, absorbing devices reduce the available radiated energy of the telephone limiting the area in which the mobile telephone may be used (i.e., reduce the coverage area of the telephone).
Consequently, there is a general need for systems and methods for lowering the SAR values of mobile telephones in all modes while limiting the impact to mechanical design of the telephone and providing maximum available radiated power when needed to furnish a good communication link in poor environments (e.g. within buildings, where large distances exist between the mobile telephone and base station, and the like).